Tuesday, March 3, 2015

Sex in the Weeds (or a Lack Thereof)

R. communis, one of the world's most
toxic plants.

Imagine you are walking down the street, minding your own
business, only to have your eyes land on a particular plant you have seen
before. You turn your head towards this chlorophyll-producing beauty and notice
its glossy palm-shaped leaves, upright growth habit, red flowers,
and….whoa….whoa….whoa! Don’t touch that thing! That’s the castor bean plant (Ricinus communis), a common invasive
species in the San Luis Obispo area, and the source of ricin. Haven’t you seen
Breaking Bad? Just four castor bean seeds can kill a full-grown Homo sapien (1).

R. communis seed.

Phew, that was a close one. Ok, so you continue on your walk, smelling a flower here, picking a blade of grass there—really enjoying the fresh air—and you start to notice more than just the occasional castor bean plant. You see ripgut brome (Bromus diandrus), little mallow (Malva parviflora), California burclover (Medicago polymorpha), and narrow-leaf plantain (Plantago lanceolata), over and over and over again. “What the bajeezers,” you think, “these same plants are everywhere!” Well, that is because they are…….WEEDS.

B. diandrus

M. parviflora

M. polymorpha

P. lanceolata

Weed scientists (yep, that’s what they are called, chuckle
chuckle) distinguish between weeds and invasive plants. Weeds are found in agricultural settings, while invasive plants exist in wild land
areas. These often despised leafy organisms have been variously defined as: “a
plant out of place”, “a plant causing economic or environmental harm”, or “a
plant that grows so luxuriantly or plentifully that it chokes out all other
plants that possess more valuable properties” (2). Noxious weeds are found in both agricultural and wild land settings. While weeds can be a nuisance, noxious weeds are so readily able to spread that they pose a high likelihood of causing economic harm and displacing natives (3).

Noxious weeds exhibit one or more reproductive
strategies that make them so successful (3):

1.)Immense
seed production: not just a hundred or even a thousand seed, we are talking
hundreds of thousands to over a million seed produced by a single plant.

2.)Unpredictable
germination rates due to dormancy (the inability to germinate under favorable
conditions).

3.)Asexual reproduction.

In California there are 251 state listed noxious weeds (4). One of the worst noxious weeds in temperate zones of the world is field bindweed (Convolvulus arvensis). It is a beautiful
specimen in the morning glory (Convolvulaceae)
family (see pic below) with a dark secret.

Field bindweed makes full use of an asexual reproduction
strategy to consistently land on lists of the world’s worst weeds (2). It
frequently invades bean, cereal, and potato fields (1), and can reduce crop yields
by 60 percent (5). In 2003, crop losses due to field bindweed were estimated to be around 377
million dollars per year in the United States alone (5). Field bindweed also
serves as an alternate host for several viruses, and a bacterium that affects
grapevines known as Pierce’s disease (Xylella
fastidiosa) (6). To top all of that, efforts to control field bindweed
chemically with roundup (glyphosate) are becoming less effective due to
multiple cellular mechanisms that lend resistance to the plant (7).

A grain crop infested with C. arvensis.

Of the many noxious weeds in California,
what is it about field bindweed that makes it such an agricultural threat? There is a one-word
answer to that: rhizomes. These are modified underground stems that store
starches and proteins and allow the plant to reproduce asexually (8). These plants are able to reproduce without pollen from the plant’s male sexual organ combining
with the ovule of a plant’s female sexual organ to form a zygote (immature
seed). No plant sex needed!

Rhizomes are full of buds that can sprout to grow new shoots
and roots. Researchers found that field bindweed can have upwards of 60 buds
per 2.5 grams of root tissue (9). A bud contains meristematic cells,
undifferentiated embryonic cells, that divide rapidly so that a plant can grow (10).
The rhizome’s buds and associated meristematic cells, as well as carbohydrate
energy reserves, allow the same plant to grow back year after year without
producing a single seed.

But wait a second, lots of plants have rhizomes. Ginger is a
rhizome. Irises have rhizomes. Even the beautiful tropical plants known as
cannas (e.g. Canna indica, see pic)
have rhizomes. If all rhizomatous plants were as successful as field bindweed,
we would be swimming in ginger and tropical flowers (actually, that sounds kind
of pleasant).

C. indica, an ornamental that spreads via rhizomes.

A C. indica rhizome.

What differentiates field bindweed from the rhizome-sprouting plants previously mentioned is its extensive root system. Doctor John
C. Frazier (1943) methodically excavated the root systems of field bindweed
plants at various stages of growth. What he found was nothing short of
remarkable. Field bindweed roots grow in a predictable manner (11):

1.)A
vertical taproot first penetrates deep into the soil.

2.)As
it grows downwards it produces multiple lateral roots. Some of these lateral
roots (known as “permanent lateral roots”, see pic), usually in the top 4-6
inches of soil, grow more extensively than others.

3.)Permanent
laterals continue their horizontal growth for 10 to 30 inches away from the
initial vertical root before dramatically bending downward themselves. While
bindweed is able to produce shoots from any part of its permanent root system,
at this bend is the point of heaviest shoot development.

An initial vertical root (P) produces multiple lateral roots. More developed laterals turn downward to form new vertical roots (S). This bend (Z) is the point of heaviest shoot development.

An analogy might clarify what is going on here: it is as
though the original shoot growth is an insect at the center of a spider web of
roots. At each location where a horizontal to vertical root bend occurs another
insect appears in this incessantly growing web (i.e. a new shoot emerges).

Frazier found that
the majority of permanent lateral roots grow away from the original vertical root
(the one at the center of the analogized web). In this way, the plant is able
to essentially move itself via spreading radial growth. In fact, an established
plant can grow 30 feet in radius in a single growing season!

Just half of a 29 week-old excavated C. arvensis root system with shoot growth.

How do the rhizomes mentioned earlier fit into this
subterranean story? Buds form all along the main vertical and horizontal roots.
Each below ground bud gives rise to a rhizome (11). Typically just a two-inch
segment of root can produce an entire new plant (1). Taking into account the conceivable
30-foot per year growth radius, and that field bindweed has an astonishing potential
rooting depth of over 20 feet, it is clear that a single plant has a whole lot
of two-inch sections. Oh yeah, and another thing, field bindweed roots are
extremely brittle (1). If a farmer cultivates an infested field in an attempt
to destroy the plant, all the farmer is doing is spreading vegetative
propagules everywhere.

Let’s say a farmer lucks out and is able to control field
bindweed with cultivation or an herbicide. Field bindweed has a back-up plan:
water impermeable seeds that can remain viable for 50 years in the seed bank.

Given the ability of just this single noxious weed to
resist multiple management strategies and spread asexually, humans are fighting a battle that they can not possibly
win. We might as well get used to seeing field bindweed and other non-native
weeds and invasive plants. They are simply masters of reproduction, even without sex.

Written by Eli Weissman

References (in order
of appearance):

1.) DiTomaso,
J.M. and H.A. Evelyn. Weeds of California
and Other Western States. Oakland: University of California, 2007. Print.